Accumulator with transport override

ABSTRACT

This invention discloses an accumulator conveyor having a powered propelling member passing through a plurality of independent operating zones arranged in tandem along the conveyor. In each zone, the propelling member is shiftable between an article propelling and an article non-propelling position by pneumatically actuated pressure rollers acting on the propelling member. A valve operated by an article detecting sensor in each zone operates to supply or exhaust fluid pressure to the actuators. An auxiliary valve system connected between a secondary pressure source and the actuators operates to shift all of the operating zones into an article propelling position thereby overriding the accumulator function of the conveyor.

DeGood Nov. 6, 1973 [76] Inventor: Maynard J. DeGood, Grand Rapids,

Mich.

[22] Filed: July 30, 1971 [21] Appl. No.: 167,617

[52] US. Cl 198/127 R [51] Int. Cl. 865g 13/071 [58] Field of Search 198/127 R, 160, 203; 246/182 A [56] References Cited UNITED STATES PATENTS 3,612,248 10/1971 Wallis 198/127 R X 3,549,880 12/1970 Seay 246/182 A 3,420,355 1/1969 DeGood et al 1981127 R 3,360,304 12/1967 Adams et al... 246/182 A X 3,156,345 11/1964 DeGood 198/127 R 3,612,247 10/1971 Pipp 198/127 R X ACCUMULATOR WITH TRANSPORT OVERRIDE Primary ExaminerEvon C. Blunk Assistant Examiner-W. Scott Carson Att0rneyPrice, Heneveld, Huizenga & Cooper 57 ABSTRACT This invention discloses an accumulator conveyor having a powered propelling member passing through a plurality of independent operating zones arranged in tandem along the conveyor. In each zone, the propelling member is shiftable between an article propelling and an article non-propelling position by pneumatically actuated pressure rollers acting on the propelling member. A valve operated by an article detecting sensor in each zone operates to supply or exhaust fluid pressure to the actuators. An auxiliary valve system connected between a secondary pressure source and the actuators operates to shift all of the operating zones into an article propelling position thereby overriding the accumulator function of the conveyor.

2 Claims, 11 Drawing Figures PATENIEmmv BIHB 3'Q7TOQl02 sum 10F 4 INVENTOR.

MAYNARD J. DE GOOD ATTORNEYS PAIENTEmuv swan 3770.102 sum aur 4 INVENTOR.

jAYNARD J. DE 6000 BY ATTORNEYS PATENTEIJMUY 6197s 3770.102

. SHEET REF 4 3o V E 1/ 74 PNEumnT/c ACTUATOR 98 14 m. 9. 13/ I56 )5 A 7 j V V CK I54 w 44 114 S0 uRc \(F 1 Z /20 8 I30 ms. ZONE ZONE ZONE I8 ZONE ZONE )0 INVENTOR.

I4 MAYNARD J. DE GOOD ms. as. jflw ATTORNEYS 1 ACCUMULATOR WITH TRANSPORT OVERRIDE BACKGROUND OF THE INVENTION This invention relates to accumulator conveyors and more particularly to an accumulator conveyor in which delivery or non-delivery of the propelling force to the article is pneumatically controlled. The invention constitutes an improvement over accumulator conveyors of the type shown, for example, in US. Pat. No. 3,420,355, M. J. De Good et al., issued Jan. 7, 1969;

The improvement of this invention lies in the fact that the conveyor utilizes compact, lightweight, simple and relatively inexpensive pneumatic means to control the movement of articles along its track. Because of the pneumatic operation in inherently provides a time delay feature between the sensingof an article and the response of the article movement control means. A further improvement lies in the provision of an override feature which permits instantaneous actuation of all operating zones regardless of the position of the sensors to effect rapid and complete clearing of the entire conveyor.

These and other objects and advantages of this invention will be readily understood by those skilled in the conveyor art upon reading the following specification and the accompanying drawings in which:

FIG. 1 is a top plan view ofa roller conveyor incorporating this invention;

FIG. 2 is a side elevational view of the conveyor shown in FIG. 1;

FIG. 3 is a sectional elevation view taken along the plane III-III of FIG. 2;

FIG. 4 is a sectional elevational view taken along the plane IVIV of FIG. 3;

FIG. 5 is a sectional elevational viewtaken along the plane V-V of FIG. 3;

FIG. 6 is a sectional elevation view similar to FIG. 3 showing the conveyor in a non-article propelling position;

FIG. 7 is an enlarged elevational view similar to FIG. 2 with portions broken away to illustrate the sensing mechanism of the invention;

FIG. 8 is a view like FIG. 7 illustrating the brake mechanism of the invention;

FIG. 9 is a pneumatic control schematic illustrating the basic pneumatic pressure system of this invention;

FIG. 10 illustrates a pneumatic control schematic of the transport override and rapid clearing feature of this invention; and

FIG. 11 is a diagrammatic view of an accumulator conveyor showing the placement of the actuators along the length of the conveyor.

In executing this invention, vertically shiftable supporting rollers for the propelling member are arranged in groups, each group being operated by a pneumatically powered actuator. The assembly is biased downwardly into a position in which the propelling member is out of article propelling position. When fluid pressure is admitted to the actuator, the bias is overcome, and the actuator shifts the support rollers upwardly into a position in which the propelling member is in an article propelling position.

An article detecting sensor operates a valve interposed between the actuator and the source of fluid pressure. In an article propelling position, the actuator is energized by the fluid pressure source through the valve. Upon the sensing of an article, the valve closes with respect to the pressure source and opens with respect to the actuator releasing the pressure therein and allowing the support rollers and propelling means to shift to an article non-propelling position.

A pivotally mounted brake mechanism connected to the actuator and operable against selected transport rollers is also provided to positively retard and stop the movement of articles on the conveyor.

Referring specifically to FIGS. 1 and 2 of the drawings, the numeral 10 indicates a conveyor track of conventional design having a pair. of side rails 12 and 13 and article supporting rollers 14 at spaced intervals therebetween forming an article supporting and conveying surface. An endless propelling member or belt 16 is located below the transport rollers 14. It will be recognized that the propelling member passes over terminal pulleys at each end of the run and has some type of conventional equipment to drive it. This equipment is not illustrated inasmuch as it is conventional in design, and many types of equipment are available for that purpose.

The belt 16 is supported for engagement with the transport rollers by support or pressure rollers 18. Each of the support rollers is rotatably mounted on a shaft 20. The rollers are supported at one end by the shaft 20 in a slot 22 (FIG. 3) formed in the side rail 12. The opposite ends of the pressure rollers are supported by the shaft on an interior support rail 24 (FIG. 3). The interior support rail 24 is provided with a plurality of equally spaced elongated openings 26 which receive the pressure roller shaft 20. The openings 26 extend vertically in the interior support rail allowing vertical movement of the shaft therein. The interior support rail 24 illustrated in greater detail in FIG. 3 is generally formed as a channel 28 partially closed at the top on the side opposite the openings 26. The channel 28 runs parallel to and is fixed to the inside of the side rail 12 such that the openings 26 are in alignment with the slots 22 in the opposite side rail 13.

An inflatable elongated tube-like member 30 is positioned inside the channel 28 and in its inflated position holds the pressure rollers in an upward or article propelling position. As illustrated, this is accomplished by means of a flat support plate 32 positioned inside the closed channel 28. When the tube is inflated, the support plate 32 is urged upwardly against the shafts 22 moving them upwardly within the confines of the slots 26. The shafts 22 on one side and the inwardly turned lip 34 on the opposite side of the closed channel confine the plate 32 and prevent it from moving out of the channel when the tube is inflated.

The tube-like member 30 illustrated in FIG. 4, may be made of a fabric reinforced neoprene rubber. The member is closed at one of its ends 36 in any convenient fashion such as by vulcanizing. The opposite end 38 is similarly closed and is provided with an inlet tube or fitting 40 which is connected by means of suitable tubing 42 through an actuator control valve 44 to a source of fluid pressure.

A plurality of article sensing assemblies 46 are positioned along the length of the conveyor 10 at predetermined intervals and provide the control means for each of the accumulating zones. Referring to FIGS. 2, 3 and 7, each sensor assembly 46 comprises a pair of spaced apart bracket members 48 and 50 mounted on pivot pins 54 which are fixed to the side rails 12 and 13 of the conveyor. At their upper ends, the brackets 48 and 50 are provided with hex holes 52 (FIGS. 2 and 7) for engagement with the shaft 55 of a sensor roller 49. In its sensing position the roller 49 is supported slightly above the level of the transport rollers 14 of the conveyor surface. The brackets extend downwardly from the roller. In the case of the bracket 50 on the side 12 of the conveyor having the interior support rail 24, the bracket curves inwardly to allow clearance for the support rail and actuator mechanism. The brackets are connected together by means of a tube 56 fixed at each of its ends to the lower ends of the brackets 48 and 50. The brackets 48 and 50, the tube 56 and the sensing roller 49 on its shaft 54 move together and form an integral sensing assembly 46. Bracket 54 is provided with an outwardly turned flange 58 which is adapted to threadably receive an adjustment screw 60. The length of the screw 60 may be adjusted for engagement with the plunger 66 of the actuator control valve 44 which will be more fully described hereinafter.

A resilient bias spring 62 is fixed at one end 64 to the conveyor side rails 12 and at its other end to the bracket 48. The spring 62 biases the sensor assembly 46'into an upper or sensing position. The upper limit of travel of the sensor roller is established by the adjustment screw 60 moving against the plunger 66 of the valve 44.

The actuator control valve 44 controls the fluid pressure supply to the pneumatic actuator 30 and also, in response to an input from the sensor, closes the input from the fluid supply source and vents the pressure in the actuator to the atmosphere. In an article propelling position the valve is open with respect to the fluid pressure source and closed with respect to the atmosphere to thereby apply the source of pressure directly to the pneumatic actuator 30.

Referring now to FIG. 5, the valve assembly 44 is seen to comprise a main body portion 70 having mounting holes 72 therein for mounting on a downwardly extending flange 25 of the interior support wall 24 by means of conventional fastening means 27. (FIGS. 3, 6 and 7).

The main body 70 of the valve is provided with a pair of communicating passageways 80 and 90..The first opening or, passageway 80 extends along the length of body 70 and is tapered outwardly and undercut at one end to form an internal valve seat 82. The inlet to the valve seat 82 has a diameter slightly larger than that of the valve seat 82 and is threaded to receive an inlet fitting 74 provided for connection to a source of fluid pressure. An ring 84, a spherical bearing or ball valve 86 and a bias spring 88 are positioned within the enlarged portion of the opening 80. TI-Ie O ring 84 is placed in the undercut groove and formsv a resilient seat surface for the ball valve 86 when it is urged forwardly (to the left as viewed in FIG. by the bias spring 88. As illustrated in the figure, the valve is closed with respect to the fluid pressure source which enters through the fitting 74.

A primary outlet 90 in the valve extends transversely of the opening 80 and opens through a side wall of the body 70. The primary outlet 90 is threaded at an enlarged end to receive a fitting 76 which may be connected by means of tubing 42 (FIGS. 3 and 6) to the pneumatic actuator 30. A piston control assembly 66 is slidably positioned in the opening 80 opposite the inlet end. The piston is provided with an enlarged head portion 94 and an extending reduced diameter shank portion 99. When the piston is positioned in the opening 80 the shank portion 99 extends along the length of the opening 80 to control the ball valve 86 as will be more fully described hereinafter. The shank is provided with a pair of spaced-apart enlarged diameter portions 92 and 93 positioned below the headand midway along the length of the shank respectively. The outer diameter of these enlarged portions is slightly less than that of the passageway 80 through which they pass. A screw 89 (shown in dotted lines) is threaded into the body and extends into the opening between the enlarged diameter portions 92 and 93. This prevents the piston from being pushed out of the opening 80 by the force of the pressure. The piston head 94 is provided for engagement with the adjusting screw 48 on the sensor bracket 50 (FIG. 2). An 0 ring 96 surrounds the enlarged portion 92 of the piston body adjacent the head and forms (when the piston is moved to the right) a pressure-tight seal between the head of the piston and the housing 70. The space between the outer diameter of the integral portions 92 and 93 and the diameter of the opening 80 provides a secondary outlet port 98 for fluid pressure.

As illustrated in the figure, the valve is in a position as would beobtained by the presence of an article on the sensor, that is, the bracket as shown in FIG. 2 and 7, is moved away from the valve, and the fluid pressure is being exhausted from the actuator to the atmosphere through the secondary outlet port 98. Fluid pressure from the actuator 30 flows back through the fitting 76, the opening 90, and through the opening 80 along the sides of the enlarged diameter portion of piston 92 and 93 where it is vented to the atmosphere between the enlarged diameter portion 92 at the secondary outlet port 98. In this position, the flow of pressure from the sourceis effectively blocked by the ball valve 86 which is held against the valve seat by the bias spring 88 and the force of the pressure itself.

When the conveyor is in an article-propelling position, the sensor 46 is'in its biased upwardly position and the fluid pressure is admitted to the actuator 30 through the valve 44 in the following matter. The bias spring 64 urges the sensor roller 49 upwardly with the sensing assembly 46 and the adjustment screw 66 on bracket 50 presses against the head of the valve 94 moving the piston assembly 66 as shown in FIG. 5 to the right. As the piston is moved to the right, the end of the shank 99 of piston 66 moves through the area of the valve seat 82 and displaces the ball valve 86 by compressing spring 88. At the same time, the O ring 96 positioned below the head 94 around 'the piston body 92 is compressed between the head 94 and valve body 70 closing the secondary outlet port 98. The fluid pressure from the source is then allowed to flow through inlet fitting 74 through ports 80 and and litting 76 to the pneumatic actuator 30 causing it to inflate and position the support rollers in anarticle propelling position.

A conveyor having a plurality ofindependent operating zones designated A through E is schematically illustrated in FIG. 11. As will be readily understood by those skilled in the art, each zone is controlled by the sensor assembly which operates to control the drive means for each zone. Normally, the sensing rollers for one zone are located adjacent to the discharge end of the next adjacent zone downstream of the conveyor in the direction of article movement. An article coming to rest on a sensor in a first zone shifts the upstream actuator to a non-propelling position and so onal ong the conveyor. To provide proper centering of the drive belt 16 along the length of the conveyor, it has been'found desirable to alternate the location of each actuator mechanism 30 along the length of the conveyor. The actuator mechanism for each adjacent zone may be p0- sitioned on opposite sides of the conveyor. Zones A, C and E maybe positioned adjacent side rail 12 while zones B and D may be positioned adjacent side rail 13. In this manner, as the belt is moved to a non-driving po- 'sition as illustrated in FIG. 3, the belt 16 in the next zone will tilt in the opposite direction thereby preventing any tendency of the belt to shift to one side of the conveyor.

Because of inertia articles will often continue moving along the conveyor even after the drive means has been shifted into an article non-propelling position. To prevent this occurrence, it is desirable to provide a brake means at each zone.

The brake mechanism illustrated in FIGS. 2 and 8 is generally indicated by the numeral 100. The brake is essentially an elongated arm-like member 102 pivotally mounted on the interior support rail 24 by conventional fastening means to form a pivot point 104. One end of the arm is bent outwardly to form a flange 106 to hold a friction pad 108 which in one position engages a transport roller 14 positioned thereabove. Bias spring 110 is connected to the opposite end 112 of the arm and to the supporting framework of the conveyor 10. When the actuator pressure is released, the support rollers are lowered to a non-propelling position; and the bias spring 110 causes the arm to pivot around the pivot point 104 thereby bringing the friaction pads into engagement with the transport rollers. The brake responds to dropping of the pressure rollers to retard or stop the movement of articles on the conveyor. Movement of the brake is controlled with movement of the actuator 30 in response to an input from the sensor 46. The arm 102 of the brake 100 is supported on a shaft 20 of a pressure roller 18 at a point midway between the pivot point 104 and the end 112 connected to the bias spring 110. As the-pressure roller 18 is moved on its shaft 20 within the slot 22 in response to movement of the actuator'a corresponding and opposite movement takes place in the brake arm. As the pressure rollers are shifted to an article non-propelling position, the end 112 of the arm is moved downwardly with the shaft by the bias spring 110 thereby causing the friction pad 108 to contact the transport roller located thereabove to stop its movement. A plurality of brake members are normally provided in each operating zone of the conveyor. Ideally, one brake will be operated by each pressure roller to control every other transport roller on the conveyor.

In normal operation of the conveyor, the articles thereon will move along the conveyor until at some point the article movement is stopped by a controlled gate (not shown) or other article movement preventing mechanism. At that time, articles accumulate until the sensor is activated thereby shifting the adjoining accumulating zone into an article non-propelling position. The articleswill accumulate in that zone and actuate its associated sensor and so on along the conveyor. When articles move along the conveyor from zone to zone without accumulating, they naturally pass over and trip the various sensors. This, however, has no effecton the actuator or pressure valve as there is an inherent time delay built into the valve. The rate atwhich the pressure is vented to the atmosphere is controlled by the difference in diameter between the passage and the enlarged diameter portions 92 and 93 on the valve shank (FIG. 5). By the time the pressure has started to vent from the actuator to the atmosphere through the secondary outlet, the article will have passed the sensor and pressure will again be applied from the source to the actuator. 1

When conveyors are operated at high speed, the inherent time delay of valve 44 may be excessive and it will become necessary to provide an auxiliary rapid exhaust valve. FIG. 9 schematically illustrates a pneumatic pressure system of this invention wherein the actuator 30 is connected to the valve 44 through a quick release exhaust valve 120. The valve 44 is connected to a source of fluid pressure 122 by line 124. The valve schematically illustrated is identical to that previously disclosed in connection with FIG. 5 with the reference numerals 74, 90, and 98 representing the input, the primary outlet, and the secondary outlet respectively.

The primary outlet from the valve is connected via line 126 through an exhaust valve through line 128 to the actuator 30. The quick release or exhaust valve 120 is a three-way valve of conventional construction having an exhaust port 130. The valve is normally biased to a closed position with respect to the input through line 126 from the valve 44 while the exhaust port is normally opened allowing the pressure in actuator to vent to the atmosphere through line 128 and exhaust port 130. When valve 44 is opened the passage of fluid pressure through line 126 overcomes the bias on valve 120 thereby allowing fluid pressure to pass through line 128 to the actuator. At the same time, the exhaust port 130 of the valve 120 is closed. Upon sensing of an article on the conveyor the valve 44 closes in the manner previouslyv described thereby blocking the fluid pressure supply and venting the outlet port 90 to the atmosphere through the secondary outlet 98. Exhaust valve 120 rapidly closes and the pressurein the actuator 30 is vented to the atmosphere through line l28'and the exhaust port 130.

- Often it becomes necessary to provide a means for rapidly clearing the conveyor surface. This is readily accomplished by simultaneously applying a fluid-pressure to each of the pneumatic actuators along the conveyor thereby causing the pressure rollers in each of the zones to move to an article propelling position. This system is illustrated iri schematic form in FIG'. 10. As contrasted to the illustration of FIG. 8, the novel rapid clearing or transport override system requires the addition of a manually controlled valve 131 connected to the source l22'a'nd a normally closed check valve 132 connected in parallel to each of the sensor actuated valves 44 at its outlet end. The manually controlled valve 131 is connected at its inlet end to the source of fluid pressure by line 134. The outlet of the valve is connected to the check valve 132 by means of line 136. One check valve is provided for each actuator and connected thereto on the primary outlet side 90 of the valve 44. The check valves are installed so that in a normal operating position there is no pressure flow from the outlet side 90 of valve 44 into line 136. When it is desired to clear the conveyor, valve 131 is opened and the fluid pressure from the source is applied through the check valve directly to the actuator without regard to the position of valve 44. In this manner, all actuators connected through check valves to line 136 will be moved to their raised position thereby shifting the drive means into an article propelling position in all zones simultaneously. A slight back flow of pressure occurs from line 136 into valve 44 through line 126 to secondary outlet 98. This is of no consequence, however as the rate at which the pressure is vented to the atmosphere is much less than the input from line 136. When the articles have cleared the conveyor, valve 131 may be closed and the actuators will begin operating in the previously described normal fashion.

This rapid clearing feature also increases the versatility of the conveyor as by simply placing the valve 131 in an open position all of the actuators in each zone are energized and the conveyor will operate as a conventional powered roller conveyor.

From the foregoing description and drawings, it will be readily apparent to those skilled in the art that the present invention provides an extremely versatile conveyor utilizing relatively simple and inexpensive component parts and which may be readily adapted for use as a conventional powered conveyor. 7

While several embodiments of this invention have been illustrated and described, it will be recognized that other embodiments and modifications of this invention incorporating the teachings hereof may be readily made in light of this disclosure. All modifications embodying the principles of this invention are to be considered as included in the appended claims unless these claims by their language expressly state otherwise.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. An accumulator conveyor having a plurality of rollers forming an article transporting track, a driven propelling member, means supporting said propelling member in driving contact with said rollers, a plurality of pneumatically powered actuator means for effecting engagement and disengagement of said propelling member from said rollers, said actuator means being arranged in tandem along said conveyor, a source of air under pressure and a first conduit connecting it in parallel to each of said actuator means, a valve interposed in said conduit between said source of air and each of said actuator means, a plurality of sensors for detecting the presence of an article on said conveyor, one said sensor operatively connected to each of said valves, each of said actuators having a pneumatically inflatable member, said conveyor characterized by a primary source of air under pressure connected to each inflatable member and a plurality of control valves, one said control valve interposed between said source and each of said inflatable members, each of said control valves being operatively connected to one of said sensor, actuation of said sensor shifting the control valve associated therewith to a closed position for effecting disengagement of said propelling member from said rollers, said conveyor characterized by a second conduit connected to said source of air and to each of said inflatable members between the control valve associated with each valve and said inflatable member, a master valve in said second conduit, said master valve when open admitting air under pressure to all of said inflatable members to thereby shift said propelling member into engagement with said rollers irrespective of whether or not any of said control valves are open for causing movement of all articles on the article propelling track, a norm ally closed check valve between said master valve and each of said inflatable members to prevent air admitted by said sensor controlled valves from entering said secondary conduit.

2. An accumulator conveyor having a plurality of rollers forming an article transporting track, a driven propelling member engageable and disengageable from said rollers; said conveyor having a plurality of sections arranged in tandem; an article detecting sensor in each section; separate and independent means in each section for bringing together and separating said rollers and propelling member for effecting drive and nondrive of said rollers, said means having an inflatable ac tuator and a valve operatively connected to a sensor for controlling the admission of a primary source of air under pressure to said actuator, said actuator in each said section positioned on an opposite side of said conveyor with respect to an adjacent section to thereby prevent lateral shifting of said propelling member as said propelling member is shifted toward and away from said rolers, said conveyor characterized by a secondary source of air under pressure connected to each of said actuators and a master valve, said master valve when open admitting air under pressure to all of said actuators simultaneously irrespective of whether some or all of said control valves are closed. 

1. An accumulator conveyor havinG a plurality of rollers forming an article transporting track, a driven propelling member, means supporting said propelling member in driving contact with said rollers, a plurality of pneumatically powered actuator means for effecting engagement and disengagement of said propelling member from said rollers, said actuator means being arranged in tandem along said conveyor, a source of air under pressure and a first conduit connecting it in parallel to each of said actuator means, a valve interposed in said conduit between said source of air and each of said actuator means, a plurality of sensors for detecting the presence of an article on said conveyor, one said sensor operatively connected to each of said valves, each of said actuators having a pneumatically inflatable member, said conveyor characterized by a primary source of air under pressure connected to each inflatable member and a plurality of control valves, one said control valve interposed between said source and each of said inflatable members, each of said control valves being operatively connected to one of said sensor, actuation of said sensor shifting the control valve associated therewith to a closed position for effecting disengagement of said propelling member from said rollers, said conveyor characterized by a second conduit connected to said source of air and to each of said inflatable members between the control valve associated with each valve and said inflatable member, a master valve in said second conduit, said master valve when open admitting air under pressure to all of said inflatable members to thereby shift said propelling member into engagement with said rollers irrespective of whether or not any of said control valves are open for causing movement of all articles on the article propelling track, a normally closed check valve between said master valve and each of said inflatable members to prevent air admitted by said sensor controlled valves from entering said secondary conduit.
 2. An accumulator conveyor having a plurality of rollers forming an article transporting track, a driven propelling member engageable and disengageable from said rollers; said conveyor having a plurality of sections arranged in tandem; an article detecting sensor in each section; separate and independent means in each section for bringing together and separating said rollers and propelling member for effecting drive and non-drive of said rollers, said means having an inflatable actuator and a valve operatively connected to a sensor for controlling the admission of a primary source of air under pressure to said actuator, said actuator in each said section positioned on an opposite side of said conveyor with respect to an adjacent section to thereby prevent lateral shifting of said propelling member as said propelling member is shifted toward and away from said rolers, said conveyor characterized by a secondary source of air under pressure connected to each of said actuators and a master valve, said master valve when open admitting air under pressure to all of said actuators simultaneously irrespective of whether some or all of said control valves are closed. 